Monolithic continuous-time MOSFET-capacitor circuits has been employed in MOS technology for analog signal processing to tolerate the process and temperature variations. However, continuous-time MOSFET-capacitor circuits are accomplished through the use of feedback around a high voltage gain operational amplifier (op amp) to achieve a well-defined voltage transfer function which is insensitive to the characteristics variation of active devices. Unfortunately, the voltage controlled op amp has several limitations which prevent high performance operation. One of the limitations is that the product of the bandwidth and the voltage gain is approximately constant. Hence, the operating frequency range of the conventional voltage op-amp based circuits is inevitably limited.
The advantages of using conveyors are high slew rate and wide bandwidth. The symbol for a current conveyor is shown in FIG. 1. There are first generation (Class I) current conveyors (CCI) and second generation (Class II) current conveyors (CCII). In CCI, equal voltages Vx and Vy appear at the X and Y terminals, and equal currents Ix and Iy flow through these two terminals respectively. The current supplied to the X terminal is conveyed to the the output terminal Z as Iz.
CCII was developed to increase the versatility of CCI in that there is no current flow in the Y terminal i.e., the Y terminal has high impedance.
A CMOS version of the CCII was reported by A. S. Sedra et.al. in "The current conveyor: history, progress and new results", IEE Proceedings, Vol. 137, Pt.G., No.2, April, 1990, pp. 78-87. In Sedra's paper, the high impedance at the Y terminal is achieved by using a separate op amp as a voltage follower. Op amps generally suffers from poor frequency response and requires relatively large capacitor as an integrator.
Another problem in continuous time IC filter design is the resistance. Resistance in IC generally cannot be controlled accurately and occupies relatively large area. If an MOSFET is used as a resistor, the voltage-current characteristic is nonlinear (or non-ohmic).
In a paper "Novel MOS resistive Circuit for synthesis of fully integrated continuous-timefilters", published in the IEEE Transactions on Circuits and Systems, Vol. CAS-33, July, 1985, pp-718-720, Z. Czarnul proposed using matched MOSFETs to replace linear resistance for continuous-time operation, and named such circuits as MOS resistive circuit (MRC). The symbol for an MRC is shown in FIG. 2. There are two voltage inputs V1 and V2; two output currents I1 and I2; two control voltages VG1 and VG2. The relationship among them is: EQU I1-I2=2K(VG1-VG2)(V1-V2) (1)
where K is a proportionality constant. The advantage of such a circuit is that no ohmic resistance need be fabricated in an IC structure. However, Czarnul's application has been limited to integrator using an op amp.